1. Field of the Invention
The present invention relates to a wastewater treatment apparatus for removing nitrogen and phosphorus contained in wastewater, and a method therefor.
2. Description of the Related Art
Nitrogen exists as total Kjeldahl nitrogen (TKN) containing ammonia (NH.sub.3) and organic nitrogen, and nitrate nitrogen (NO.sub.X) in wastewater. In order to biologically remove nitrogen components contained in wastewater, conversion into nitrate nitrogen must be preceded. The nitrate nitrogen is emitted into air after being converted into gaseous N.sub.2 through denitrification by microorganisms. The denitrification by microorganisms requires presence of organic matter and inhibited by dissolved oxygen (DO) in the wastewater.
Meanwhile, in order to remove phosphorus from wastewater, after releasing phosphorus by microorganisms in anaerobic conditions, the released phosphorus must be taken up by microorganisms in aerobic conditions. Particularly, conditions for releasing phosphorus by microorganisms in anaerobic conditions require no nitrate nitrogen present. This is because the release of phosphorus is inhibited with the presence of nitrate nitrogen. That is, a reaction tank must be maintained in anaerobic conditions for the release of phosphorus, and in anoxic conditions for denitrification of the nitrate nitrogen.
In a conventional biological treatment for removing nitrogen and phosphorus from wastewater, wastewater to be treated is usually provided into a reaction tank, and then release of phosphorus and denitrification of nitrate nitrogen by microorganism are performed in anoxic conditions. Then, nitrification and "luxury uptake" of phosphorus are performed in an aeration tank. Here, a return sludge was provided to the reaction tank from the end of the aeration tank or a settling tank. Here, denitrification efficiency in the reaction tank is dependent on the content of nitrate nitrogen contained in the return sludge. Also, the removal efficiencies of nitrogen and phosphorus change according to the content of the organic matter required for the denitrification and phosphorus release reaction. Thus, when intending to remove nitrogen and phosphorus contained in the wastewater using a reaction tank instead of a multi-compartment reaction tank, removal efficiencies of nitrogen and phosphorus are different, thus extending retention time in the reaction tank and there is a limitation in removing high concentration of nitrogen and phosphorus.
Also, when wastewater temperature is low as in winter, nitrification reaction is inhibited, so nitrate nitrogen required for denitrification is not produced. In this case, the oxygen content must be supplemented to generate nitrate nitrogen. If the concentration of dissolved oxygen in the reaction tank is increased, an aerobic/anoxic condition cannot be maintained and nitrogen and phosphorus removal efficiencies are lowered.